Higher free-living activity energy expenditure is strongly associated with lower risks of coronary heart disease, cancer incidence, falls, and mortality among older adults. It is unknown, however, how activity energy expenditure can protect older adults from physical disability and premature mortality. The importance of mitochondrial function to the rate of progression of age-related diseases such as cancer, diabetes, and neurodegeneration has become increasingly apparent in recent years. Yet little is currently known about the role of mtDNA mutations in human bioenergetics. Our primary aim is to assess the association of mtDNA polymorphisms and heteroplasmy with active energy expenditure and resting metabolic rate in the elderly. We will carry out our primary aim in a test of healthy samples from the Health, Aging and Body Composition (Health ABC) Study cohort. Active energy expenditure and resting metabolic rate were measured in 1998-1999 in 302 high-functioning, community-dwelling older adults (aged 70-82 years). We will use the recently developed Affymetrix Mitochondrial Resequencing Array v2.0 (MitoChip) to sequence the entire mtDNA genome of 200 study participants from the highest and lowest tertiles of free-living activity energy expenditure (>770 kcal/d and <521 kcal/d, respectively). The results of this study may directly link mtDNA mutations with energy expenditure and metabolic rate in the elderly and provide a mechanism by which specific mtDNA mutational events contribute to bioenergenic decline and subsequent mortality. The results of our study may have profound biological and clinical significance. Identifying genetic variants that influence energy expenditure and metabolic rate could eventually lead to interventions that prolong the productive and healthy years of human life. PUBLIC HEALTH RELEVANCE: Our primary aim is to assess the association of mtDNA polymorphisms and heteroplasmy with active energy expenditure and resting metabolic rate in the elderly. We will carry out our primary aim in a test of healthy samples from the Health, Aging and Body Composition (Health ABC) Study cohort. We will use the recently developed Affymetrix Mitochondrial Resequencing Array v2.0 (MitoChip) to sequence the entire mtDNA genome of 200 study participants from the highest and lowest tertiles of free- living activity energy expenditure. The results of this study may directly link mtDNA mutations with energy expenditure and metabolic rate in the elderly and provide a mechanism by which specific mtDNA mutational events contribute to bioenergenic decline and subsequent mortality.